(1) Field of the Invention
The present invention relates to an image forming apparatus that primarily transfers a toner image onto an intermediate transfer member, and then secondarily transfers the toner image onto a recording sheet using a secondary transfer roller. The present invention particularly relates to an art for cleaning a circumferential surface of the secondary transfer roller.
(2) Related Art
An image forming apparatus employing a so-called intermediate transfer system performs image formation by primarily transferring a toner image formed on an image carrier onto an intermediate transfer member such as an intermediate transfer belt, and then secondarily transferring the toner image onto a recording sheet using a secondary transfer roller. According to such an image forming apparatus, a surface of the secondary transfer roller becomes stained because a toner adhered to a non-image area of the image carrier adheres to the surface of the secondary transfer roller via the intermediate transfer member, or a toner that has not been transferred onto a recording sheet due to, for example, a paper feeding error adheres to the surface of the secondary transfer roller.
If this stain on the surface of the secondary transfer roller is left unaddressed, a rear surface of the recording sheet will become stained, and it might occur that transfer efficiency decreases and as a result defective transfer is caused.
Accordingly, the above image forming apparatus regularly cleans the secondary transfer roller by applying a voltage having the same polarity as that of the toner to the secondary transfer roller, and reversely transferring the toner adhered to the surface of the secondary transfer roller onto the intermediate transfer member (See Japanese Laid-Open Patent Application Publication No. 2002-182498, for example).
Here, there has been conventionally used an image forming apparatus in which, for the purpose of reducing manufacturing costs thereof, a power supply of a developing bias voltage (hereinafter referred to simply as “bias voltage”) is used for performing-cleaning of the secondary transfer roller instead of additionally providing a power supply apparatus dedicated to performing the cleaning.
Generally, in order to keep a constant toner concentration, a bias voltage is controlled by a known bias voltage control apparatus so as to be within a range of approximately −300 V to −700 V. When a bias voltage is applied to a secondary transfer roller in order to clean the secondary transfer roller as described above, the bias voltage control apparatus controls the bias voltage to be a voltage necessary for performing the cleaning (approximately −500 V).
By the way, it has been conventionally known that when an original bias voltage is controlled to be a voltage necessary for cleaning a secondary transfer roller, or when the voltage necessary for performing the cleaning is restored to the original voltage, this control of the voltage causes a phenomenon that a toner on a surface of a developing roller adheres to an image carrier. In order to prevent the adhered toner (hereinafter referred to as “fog toner”) from being further transferred onto an intermediate transfer member and moving to a secondary transfer position, and as a result adhering to a surface of the secondary transfer roller, the following action is taken. The secondary transfer roller is kept separated from the surface of the intermediate transfer member till a part of the intermediate transfer member to which the fog toner has adhered passes by the secondary transfer position, and then an operation of cleaning the secondary transfer roller is performed.
However, according to the above conventional art, the fog toner needs to be caused to pass by the secondary transfer position before and after performing the cleaning operation. Accordingly, it is inevitably necessary to take periods for keeping the secondary transfer roller separated from the intermediate transfer member before and after performing the cleaning operation. This delays completion of the cleaning operation, and as a result, there occurs a problem that execution of subsequent image forming jobs always delays.